1. Field of the Invention
The present invention relates to a sheet conveying apparatus for separating and feeding sheets one by one to a predetermined position, and more particularly to a sheet conveying apparatus applicable to an image forming apparatus such as a copying machine and a laser beam printer.
2. Related Background Art
There are proposed various types of conventional sheet conveying apparatus for automatically conveying the sheet to an image reading portion of an image forming apparatus such as a copying machine, in which a plurality of sheets resting on a rest stand are separated one by one in a separating portion and conveyed to the image reading portion of the image forming apparatus, and which is provided with a means for reversing a front surface and a back surface of the sheet or the like.
A flow of the original in the separating portion in the conventional sheet conveying apparatus will now be described briefly with reference to FIGS. 6 and 7 (cross-sectional view and a view as viewed in a direction indicated by the arrow A). First of all, a stack of sheet Sa rested on a rest stand 11a is picked up one by one from a top portion of the sheet stack Sa by a feed means such as a feed roller 1a that stands by above the sheet stack Sa and is fed toward the separating means downstream thereof. A fixed guide 6a for smoothly guiding the sheet to the separating means is provided in a position where it does not interfere with the up-and-down movement of the feed roller 1a between the feeding means and the separating means.
The sheets that have been fed without being separated one by one by the feed roller 1a (double feed) are separated one by one by the separating means (that is composed of a separating conveying roller 3a and a separating pad 4a) located downstream of the feed roller 1a and conveyed to a predetermined image reading position.
The operation of the feed roller 1a and the separating conveying roller 3a will now be described.
First of all, the separating conveying roller 3a rotates about a shaft 5a in a sheet conveying direction (clockwise direction in FIG. 6) using a motor Ma as a drive source. The feed roller 1a is subjected to the drive transmission through a timing belt 7a and rotates about a shaft 13a in the sheet conveying direction (clockwise direction in FIG. 6) using the same motor Ma as the separating conveying roller 3a as a drive source. The up-and-down movement of the feed roller 1a is performed by means of the same motor Ma through arms 2a rotatable about the above-described shaft 5a. When the feed roller 1a is brought into contact with the sheet stack and a torque (pressure) equal to or more than a predetermined level in a direction in which the feed roller 1a presses the sheet stack is applied, a slide is generated between the shaft 5a and the arms 2a by spring clutches 12a each engaging between the shaft 5a and the arm 2a, to thereby interrupt the lowering motion of the feed roller 1a (see FIG. 12).
However, if the above-described arrangement is taken as in the conventional case, there is a fear of the generation of the following defects.
Although the fixed guide 6a for guiding the sheet from the feed roller 1a to the separating conveying roller 3a is provided in the position where it does not interfere with the up-and-down movement of the feed roller 1a, the same fixed guide could not be provided in the position (space between the arms 2a) where it interferes with the up-and-down movement in consideration of the swing (upward and downward movements) of the arms 2a, and there is a fear that the sheet could not be guided smoothly between the arms 2a. As a result, the sheet is brought into abdominal abutment with the separating conveying roller 3a (the phenomenon in which the leading end of the sheet collides against a portion other than the vicinity of the separating nip and is not led to the nip (see FIG. 11)), the leading end portion of the sheet is raised and damaged in the worst case.
In order to solve such a problem, as shown in FIGS. 8 and 9, an arrangement is proposed in which a fixed guide 14a (that is a discrete member from the fixed guide 6a but integral with the arms 2a) is provided between the arms 2a to prevent the abdominal abutment to the separating conveying roller 3a. However, if such an arrangement is taken, there is a disadvantage that due to the sheet feeding structure of the feed roller 1a, in the case where a plurality of sheets are fed toward the separating conveying roller 3a at once, the plurality of sheets are constrained by the guide 14a so that no sheet could be fed to the separating conveying roller 3a (non-feed). Furthermore, in order to avoid the above-described abdominal abutment and non-feed, as shown in FIG. 10, it is proposed to provide a structure in which the above-described fixed guide 14a is rotatable about a pivot xe2x80x9cbxe2x80x9d on each of the arms 2a. However, even in such a structure, in the case where a plurality of sheets with their leading end folded are introduced to the separating conveying roller 3a, there is a fear that it is brought into abdominal abutment and the guide 14a is forced up (in a direction indicated by an arrow in FIG. 10). This is insufficient as a countermeasure.
Moreover, in a practical point of view for shortening a processing time concomitant with the high speed operation, since the stack displacement of the sheets on the feed tray 11a and the stack introduction (a state where a number of sheets are introduced to the separating means in the form of a stack) are generated due to the shock upon the up-and-down movement of the feed roller 1a by the arms 2a, the up-and-down movement is not performed in many cases in the sheet feeding operation (i.e., the feed roller is always pressed to the sheet during the operation). This is the case in the above-described conventional case. Then, in the conventional structure, since the force is applied to the feed roller 1a in a direction in which the sheet stack is pressed to thereby keep a conveying force, the force in addition to a self-weight of the feed roller is applied to the feed roller 1a by the spring clutch 12a that is the force applying means to compensate for the conveying force. Namely, the conveying force is kept by means of the force of the spring clutch 12a and the gravitational force (the self-weight) of the feed roller 1a upon the forward drive of the motor Ma (in which the feed roller 1a, the separating conveying roller 3a and the arms 2a work in the direction indicated by the arrow in FIG. 6). However, upon the interruption of the drive of the motor Ma, the force which is applied to the feed roller 1a by the spring clutch 12a is released and the bias is attained only by means of the gravitational force. Accordingly, in some cases, the feed roller 1a is jumped up by the force of restitution of the elastic deformation of the feed roller 1a or the restitution force of the elastic deformation of the spring clutch that is larger than the restitution force of the feed roller 1a immediately after the interruption of the drive, so that the feed roller 1a stays in a position somewhat away from the uppermost surface of the sheet. Then, upon the next sheet feeding operation, the feed roller 1a is lowered again downwardly so that the shock is applied to the sheet and the stack displacement and the stack introduction into the separating means would occur. Also, in the case where the separating means is not provided, the stack introduction would occur in the conveying means downstream of the conveying portion so that there is a fear that the faulty sheet conveyance would occur.
Accordingly, in view of the foregoing problems, an object of the present invention is to stabilize the feed condition of the sheet, and more particularly to prevent the stack displacement on a rest stand and the stack introduction.
In order to attain this and other objects, a sheet feeding apparatus according to the present invention is characterized by comprising:
a rest stand for resting a sheet; and
feeding means for feeding the sheet on the rest stand,
in which the feeding means swings about a rotary shaft to be movable up and down,
the feeding means abuts against the sheet on the rest stand by force applying means provided on the rotary shaft with a predetermined force, and
a force of the force applying means is smaller than a gravitational force (self-weight) of the feeding means.
In the foregoing arrangement, since the force of the force applying means is set to be smaller than the gravitational force of the feeding means, the fine up-and-down movement of the feeding means during the feeding operation is reduced to prevent any shock, any stack displacement of the sheet on the rest stand, and the stack introduction without fail.